Various methods have been utilized to strengthen glass articles without the addition of coatings or laminates. Generally, these techniques involve promoting an increase in glass strength by altering the surface of the glass articles to be strengthened. This is usually achieved by creating compression in only the surface of glass articles which results in the development of tensile stresses. Strengthening is achieved, because such surface compressive forces must be overcome to fracture the glass.
One conventional method of strengthening glass is tempering, where a glass article is heated and then quickly chilled so that the outside portion of the article is cooled and shrinks before the interior. As the interior cools and shrinks, the surface zone is placed in compression. See U.S. Pat. No. 3,490,984 to Petticrew et al.
Another known glass strengthening method involves the substitution or exchange of smaller ions of an alkali metal from an external source for larger ions of a different alkali metal at the surface of the glass. It has also been suggested, in U.S. Pat. No. 3,356,477, that sodium ions can be replaced with ionized potassium salts. As in tempering, compression is developed at the surface of the article. However, ion exchange achieves such compression by changing the surface composition of the glass from that of the interior.
Variations of the ion exchange strengthening technique are disclosed by U.S. Pat. No. 3,743,491 to Poole et al., which sprays glass bodies with tannic chloride or titanium or zirconium compounds prior to ion exchange of sodium with potassium, and U.S. Pat. No. 4,872,896 to LaCourse et al., which effects ion exchange in conjunction with the application of microwave radiation.
All such ion exchange processes--whether ions in the glass surface are replaced by smaller ions or by larger ions--require the ions in the glass surface to be exchangeable. Soda-lime glasses contain replaceable sodium ions and, therefore, are commonly strengthened by ion exchange. In general, however, such processes have limited utility, because they do not greatly strengthen glass which lacks exchangeable sodium ions.
To overcome this problem, U.S. Pat. No. 3,697,242 to Shonebarger strengthens glass by crowding lithium and/or potassium ions into the surface of glass without counterbalancing ion removal. Such addition causes ions of the added compounds to migrate into the surface of the glass and establish a compression zone that strengthens the glass.
Yet another glass strengthening process involves contacting glass articles with gaseous sulfur dioxide, as disclosed by U.S. Pat. Nos. 3,451,796 to Mochel and 4,341,543 to Andrus et al. Such an approach, however, requires separate handling and treatment systems to permit utilization of such poisonous, corrosive gases.
One problem with all processes of strengthening glass by compression is the inability to control surface layer thickness. Generally, the compressed surface layer can have a thickness ranging up to a few millimeters which severely limits the size of articles that can be strengthened. Thus, thin glass sheets, glass fibers, and other articles with small dimensions cannot be strengthened, because the depth of the induced surface compressive layer may exceed the dimensions of the glass article. This has severely limited the size of articles which can be strengthened. In addition, such control problems limit the usefulness of surface compression techniques in treating articles with different regions of varying thickness, because uniform strengthening is particularly difficult to achieve in different regions.
A concurrent development in the glass industry has been the photonucleation and crystallization of aluminosilicate glass articles with electromagnetic radiation. Upon heat treatment between the glass's annealing point (i.e., the temperature which, if held a short time, usually about 1 hour, causes most of the stresses in the glass to be relaxed) and its softening point (i.e., the temperature at which the glass starts to flow at a viscosity of about 1.times.10.sup.+7.6 poise), crystallization occurs in exposed areas. The aluminosilicate glass treated according to this procedure also contains alkali metal- or alkaline earth metal-oxides, usually lithium oxide, together with a cerium dioxide optical sensitizer and a nucleating agent, such as a salt of gold, silver, copper, and palladium with gold and/or silver.
Upon exposure of the glass to ultraviolet radiation, Ce.sup.+3, formed when the initially-present cerium dioxide undergoes thermoreduction to Ce.sub.2 O.sub.3 during melting of the glass, is converted to Ce.sup.+4 and liberates an electron according to the following reaction: EQU Ce.sup.+3 +h.sub..gamma. .fwdarw.Ce.sup.+4 +e.sup.-
The liberated electron diffuses to nearby silver ions where it reacts as follows: EQU Ag.sup.+ +e.sup.- .fwdarw.Ag.degree.
Subsequent heat treatment causes agglomeration of Ag.degree. atoms which forms Ag colloids dispersed throughout the glass. These colloids act as nucleating sites for crystal growth during heating at temperatures between the annealing and softening points of the glass. Specifically, in lithium aluminosilicate glass, such heat treatment causes lithium metasilicate or other crystalline phases to form on the silver colloids. If gold, copper, or palladium with gold and/or silver are used instead of silver, they function similarly. Crystallization occurs only in the areas exposed with ultraviolet radiation. The crystallized glass is then cooled slowly.
The use of this process to produce colored glass is disclosed in U.S. Pat. Nos. 2,515,275, 2,515,937, 2,515,940 ("Stookey '940"), 2,515,941 ("Stookey '941"), 2,515,942, 2,515,943 ("Stookey '943"), and 2,971,853 ("Stookey '853") all to Stookey and British Patent Nos. 635,649 and 752,243 ("Stookey '243") to Stookey. See also S. D. Stookey, "Coloration of Glass by Gold, Silver, and Copper," J. American Ceramic Society, Vol. 32, No. 8 (1949) and R. J. Futato and R. H. Doremus, "Nucleation in Photosensitive Gold Ruby Glass," J. American Ceramic Society, Vol. 63, Nos. 3-4 (1979). As shown in Stookey '943, such coloration takes place throughout the bulk of the glass where it has been exposed. In Stookey '940 and Stookey '941, it is recognized that coloration occurs when such glass is exposed due to the formation of some alkali metal disilicate crystals throughout or at least substantially into the bulk of the exposed glass. In Stookey '243 and Stookey '853, lithium aluminosilicate glasses are treated, and it is suggested that .beta.-spodumene crystals are produced in addition to lithium metasilicate crystals. However, these crystals are present as a crystalline solution of .beta.-spodumene and quartz.
U.S Pat. No. 3,445,209 to Asunmaa also discloses the formation of crystals in glass by exposure with radiation.
It has also been known to chemically machine glass which has been treated with ultraviolet radiation, heated, and cooled. This is achieved by treating the cooled glass with a 5% HF solution to remove, by dissolution, the low chemical durability lithium metasilicate phase present through the bulk of the glass in exposed areas. As a result, a solid glass with various desired patterns remain. See Stookey '243 and J. D. Stookey, "Chemical Machining of Photosensitive Glass," Industrial and Engineering Chemistry, Vol. 45, No. 1, pp. 115-118 (1953).